Since some time for now Mozilla has been working on new user interface for Firefox and Australis is the new theme they’re going to use for Firefox, you may’ve heard and seen few mockups of upcoming Firefox UI and an add-on is available on AMO as well, it seems Mozilla UI team is close to completing Australis Tab shape for Firefox (windows only), and you can see Australis tab shape for Firefox by downloading a nightly build from here.

These tabs are curved, once Firefox gets Australis theme you’ll see panel based application menu and other significant changes from existing version of Firefox, so Firefox menu button which introduced in Firefox 4 to be ditched and you’ll get panel based app menu at the end of Firefox navigation bar just like to Chrome’s settings menu.

These tabs appears pasted on tab bar like live bookmarks, when you hover your mouse pointer over other tabs those tabs appear in curved shape too.